The present invention relates to a semiconductor device having a multi-layer structure composed of insulating films and at least a conductive film provided on a semiconductor substrate.
A semiconductor device is manufactured through a sequence of process steps such as impurity diffusion step, oxidation step, chemical vapour deposition step, photolithography step, etching step, and so on. Through these process steps, multi-layer structures of conductive films such as wirings or electrodes and insulating films interposed between conductive films are formed on a semiconductor substrate. The multi-layer structure is very thick and high with a sharp angle at its corner. Therefore, a wiring especially of aluminum provided on the multi-layer structure is apt to break off at the corner. For avoiding this problem, in the prior art, a phosphosilicate glass (hereinafter referred to as PSG) film having a high phosphorus concentration of, e.g., 12 to 15 weight percent is formed by chemical vapour deposition on the substrate such that it covers the multi-layer structure, and is subjected to a heat treatment at 950.degree. C. to 1000.degree. C. so that the upper surface of the PSG film becomes smooth above the corner. The aluminum wiring formed on the smooth upper surface of the PSG film is hardly broken off. However, another problem occurs in this method. Namely, when the semiconductor device is of the plastic package type, water is easily penetrated and the aluminum wiring is corroded by phosphoric acid which is produced by the water and phosphorus contained in a high concentration in the PSG film. If a PSG film having a low concentration of phosphorus is used, the corrosion problem is reduced. However, in this case, the upper surface thereof is not sufficiently smoothed by the heat treatment, and the above-mentioned break off problem of the wiring cannot be solved. Recently, a borophosphosilicate glass (hereinafter referred to as BPSG) film containing boron as well as phosphorus has been employed in place of the PSG film. The BPSG film is disclosed, for example, in "RCA Review" Vol. 43, September, 1982, pp. 423 to pp. 457, by Werner Kem and George L. Schnable. The BPSG film solves both of the break off problem and the corrosion problem mentioned above to a considerable extent.
However, the inventor of the present invention has discovered the following problem. The BPSG film is formed over a wiring layer or over the multi-layer structure and is subjected to the heat treatment to smooth the upper surface thereof, that is, to "reflow" the BPSG film for the purpose of protecting another wiring layer formed on the upper surface from being broken above the corner. A polycrystalline silicon layer is then formed on the BPSG film, and an impurity such as phosphorus or boron is diffused into the polycrystalline silicon layer. In this case, after the impurity diffusion step, wrinkles appear on the surface of the polycrystalline silicon layer. This phenomenon means that strains are produced in the polycrystalline silicon layer. This phenomenon must be avoided because the wiring or electrode pattern to be formed by the polycrystalline silicon layer may be otherwise partially moved in the subsequent process step of the device such as a thermal step. Moreover, precise patterning of the polycrystalline layer becomes impossible because the etching speed of the layer may be changed at the portions where the wrinkles exist.